The invention generally relates to imaging systems, and particularly relates to systems and methods for producing high quality images using light modulators such as diffractive light modulators.
Systems for producing high quality images using light modulators typically include an illumination source for producing an illumination field, a light modulator for receiving the illumination field and for producing a modulated illumination field, and imaging optics for directing the modulated illumination field toward an imaging surface. For example, U.S. Pat. No. 6,433,934, the disclosure of which is hereby incorporated by reference, discloses an imaging system that includes an illumination source, a field lens system, a light modulator, imaging optics and an imaging drum for supporting recording media.
The illumination field may include light of a plurality of frequencies and the diffraction of the illumination field by the light modulator will occur at different angles for the different frequencies. The diffraction in discrete angles occurs only for diffractive elements having an infinite number of diffractive units. As the number of interacting diffractive units is increased, the angular distribution of the intensity of the diffracted fields increases and the intensity distribution in the Fourier plane increases.
With reflective light modulators, the first order reflection may be used for imaging. A second order (and to a lesser extent third and fourth order etc.) reflected fields may produce unwanted illumination that is filtered out in the Fourier plane by a slit diaphragm. For example, as shown in FIG. 1, such a conventional imaging system includes an illumination source 10 that produces illumination that is directed by an objective lens 12 (e.g., f=14.5 mm), a plano convex lens 14 (e.g., f=220 mm), and a cylindrical lens 16 (e.g., f=100 mm) toward a mirror 18, which in turn directs the illumination through a corrected objective lens 20 toward a light modulator 22 (e.g., a grating light valve). The illumination is modulated by the light modulator 22 and reflected back through the lens 20 and through a Fourier plane, to an imaging system lens 24, which directs the modulated illumination toward an imaging surface 26 at the imaging plane. A slit diaphragm 28 is positioned in the Fourier plane. The positive and negative first order reflected signals are passed through the slit diaphragm 28, while other order reflected signals (e.g., second, third etc.) are blocked by the slit diaphragm 28.
Conventional Fourier diaphragms, however, cannot be spatially modulated to accommodate modulated signals having different frequencies, for example, time division multiplexed multi-frequency signals such as red, blue, green illumination. This results in a decrease in the resolution in the image plane and a reduction in intensity of the imaging field for systems including multi-frequency illumination sources. This is due, at least in part, to the presence of unwanted illumination (or side lobes) in locations near individual illuminated pixels or small groups of illuminated pixels.
There is a need, therefore, for a multi-frequency imaging system having improved filtering in the Fourier plane.
The invention provides an imaging system that includes an illumination source, a modulator and a variable Fourier diaphragm. The illumination source produces a first illumination field having a first frequency and a second illumination field having a second frequency. The modulator receives the first illumination field and the second illumination field, and produces a first modulated illumination field and a second modulated illumination fields. The variable Fourier diaphragm selectively passes one of the first modulated illumination field and the second modulated illumination field. In various embodiments, the modulator is a grating light valve and the imaging system may vary the variable Fourier diaphragm responsive to changes in the period of the spacing of ribbons in the grating light valve.